BUNDLER_HOME env var

[svoop]

Since 1.14.x, Bundler tries to create a fallback home directory underneath Dir.tmpdir in case the user home read from Bundler.rubygems.user_home (doing File.expand_path "~") is not writable. This is causing problems in certain scenarios such as #5371 and #5363.

How about adding an environment variable $BUNDLE_HOME:

• When set to a path, the path is used instead of Bundler.rubygems.user_home. Good for deployments to readwrite FS where the regular user home is not writable due to security restrictions.
• When set to "", no fallback home directory is created and Bundler does not attempt to read the global config. Good for deployments to writeonly FS (e.g. Docker).

[segiddins]

As I posted in #5371, bundler should swallow the error when attempting to read the global config anyways

[colby-swandale]

This has been added in #6024 which is part of 1.17.0.pre.2